I. Field of the Invention
The present invention relates to data transmissions. More particularly, the present invention relates to method and apparatus for rotating a phase of a modulated signal.
II. Description of the Related Art
In many communications systems, data is processed, modulated, and conditioned prior to transmission to a receiving device. The data processing can include encoding for compression, coding for error detection/correction, covering to provide orthogonal channels, spreading to spectrally disperse the data, and others. The processed data is then modulated with one or more carrier signals to generate a modulated signal more suitable for transmission. Various modulation schemes can be used such as quadrature phase shift keying (QPSK), offset QPSK, quadrature amplitude modulation (QAM), frequency modulation (FM), amplitude modulation (AM), and others. The modulated signal is typically further conditioned (e.g., buffered, amplified, filtered, and so on) prior to transmission.
The signal processing is performed by a number of circuit elements in a transmit signal path, and some of these elements can introduce distortions to the signal. For example, the non-linearity of a circuit element (e.g., amplifier, mixer) can generate intermodulation distortions. Moreover, the circuit elements can introduce phase rotation in the modulated signal. Conventionally, these distortions are maintained at an acceptable levels by proper design of the circuit elements.
To improve performance and enhance system capacity, the transmit signal path can include circuit elements that can be selectively controlled based on the operating condition of the transmitting device. The transmit signal path may include, for example, one or more variable gain elements having gains that can be adjusted to provide a variable output transmit power level. The transmit signal path may further include active circuit elements having bias currents that can be adjusted to reduce power consumption while providing the required (e.g., linearity, bandwidth) performance.
The characteristics of the modulated signal can change as the variable circuit elements in the transmit signal path are adjusted. Specifically, the phase of the modulated signal can rotate as the circuit elements are adjusted. When the circuit elements are adjusted in a step-wise fashion, the modulated signal can include phase discontinuities, which can cause performance degradation. Such phase transitions can be tracked at the receiver. However, this places a constraint on the bandwidth of the phase estimate. In order to operate at very low signal-to-noise ration (SNR), it is desirable to use narrowband estimators (i.e., having long time constant) that, in turn, have poor tracking performance.
Thus, techniques that can rotate the phase of the modulated signal to compensate for the phase rotation due to adjustments of circuit elements associated with the transmit signal path are highly desirable.